Method of controlling the hydrocarbon dew point of a gas stream

ABSTRACT

The present invention relates to a method of controlling the hydrocarbon dew point of a gas stream in order to prevent condensation of the gas stream in subsequent gas transmission and distribution systems. The gas stream is passed in heat exchange relationship with an expanded refrigerant stream thereby cooling the gas stream and liquifying only those condensible components in said gas stream required to obtain a desired residue gas dew point. The cooled residue gas is then passed in heat exchange relationship with the refrigerant stream after the refrigerant stream has been compressed thereby condensing the refrigerant stream and heating the residue gas stream well above its dew point.

United States Patent Rosen Aug. 28, 1973 [5 METHOD OF CONTROLLING THE2,909,905 10/1959 Mitchell 62/23 HYDROCARBON DEW POINT OF A GAS3,076,318 2/ 1963 Becker 62/23 2,538,664 l/195l Benz 62/50 STREAM [75]Inventor: Ward F. Roan, Oklahoma City,

Okla.

[73] Assignee: Black, Sivalls & Bryson, Inc., Kansas City, Mo.

[22] Filed: Feb. 10, 1969 [21] App]. No.: 797,960

[52] U.S. Cl 62/21, 62/1 1,62/40, 62/50 [51] Int. Cl. F25j 3/00, F25j3/06 [58] Field of Search 62/9, 11, 23, 40, 62/21, 37, 50; 208/351, 352,368

[56] References Cited UNITED STATES PATENTS 2,134,699 11/1938 Brewster62/40 2,258,015 10/1941 Keith 2,265,558 12/1941 Ward 2,274,094 2/1942Rupp 62/23 Primary Examiner-Norman Yudkoff Assistant Examiner-A. F.Purcell Attorney-Dunlap, Laney, Hessin & Dougherty 5 7] ABSTRACT Thepresent invention relates to a method of controlling the hydrocarbon dewpoint of a gas stream in order to prevent condensation of the gas streamin subsequent gas transmission and distribution systems. The gas streamis passed in heat exchange relationship with an expanded refrigerantstream thereby cooling the gas 6 Claims, 2 Drawing Figures METHOD OFCONTROLLING THE HYDROCARBON DEW POINT OF A GAS STREAM BACKGROUND OF THEINVENTION stream are liquified to produce a residue gas stream having adesired hydrocarbon dew point.

2. Description of the Prior Art Natural gas produced from oil and gaswells normally comprises a mixture of hydrocarbon components havingvarying boiling points. That is, a gas stream produced from a wellcontains a mixture of hydrocarbon components which exist in the vaporphase at the particular pressure and temperature levels at which thewell is produced. If the gas stream pressure is changed or thetemperature is decreased, or both, some of the hydrocarbon componentscontained in the gas stream are liquified or condensed. The temperatureat which some of the components of a gas stream comprised ofhydrocarbons will condense at a particular pressure level is known asthe hydrocarbon dew point of the gas stream.

In the natural gas industry, a number of oil and gas wells are commonlyproduced'with the gas streams from all the wells passing into a commonpipeline for transmission and distribution at a point remote from theoil feld. As' the gas passes throughthe transmission pipeline, thepressure of the gas drops due to friction,-

and particularly duringthe winter months, the temperature of the gas islowered due to atmospheric conditions. Thus, condensiblecomponentscontained within the gas stream passing through the pipelineare condensed.'The condensation of such components in a gas transmissionsystem is detrimental in that the flow of gas through the system isimpaired, and, if not removed prior to distribution of the gas todomestic users, explosions andfires may result. w

Many various methods and apparatus have been developed for removingcondensible components from gas streams-prior to the gas entering thetransmission pipe line. Some examples of these methods are adsorption ofcondensible components from gas streams on beds of dry desiccant such asactivated carbon, silica gel, etc., adsorption of condensible componentsby a liquid desiccant such as naphtha or kerosene, and condensation ofcondensible components through expansion of the gas stream. Thesemethods commonly in- SUMMARY OF THE INVENTION The present inventionrelates to a method of controlling the hydrocarbon dew point of a gasstream comgas stream are liquified and a residue gas having a desiredhydrocarbon dew point remains. The residue gas is separated from theliquified condensible components, and the expanded refrigerant stream iscompressed. The residue gas stream is then heated with the compressedrefrigerant stream to a temperature level above the dew point of saidresidue gas stream, and the residue gas stream is conducted to adistribution systern.

It is therefore a general object of the present invention to provide amethod of controlling the hydrocarbon dew point of a gas stream.

A further object of the present invention is the provision of a methodof controlling the hydrocarbon dew point of a gas stream wherein onlythose components which will condense at conditions to be encounteredwithin the transmission or distribution system are removed therebymaintaining the heating value of the residue gas as high as possible. 5

Yet a further object of the present invention is the provision of amethod of controlling the hydrocarbon dew point of agas stream which isrelatively inexpensive to carry out. g A

Other objects and advantages of the invention will be evident from thefollowing detailed description when read in conjunction with theaccompanying drawings which illustrate the invention.

DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates, in diagrammatic form,apparatus for carrying out the method of the present invention, and

FIG. 2 is a graph illustrating temperature and pressure conditions atwhich condensation occurs for typical gas streams.

DESCRIPTION OF THE PREFERRED- EMBODIMENT Referring to the drawings, andparticularly to FIG. 1, conduit 10 leads an inlet gas stream to heatexchanger 7 12 wherein the gas stream is cooled by exchange of volveconverting a high percentage ofthe components in the gas stream to theliquid phase and separating them from the gas stream leaving a residuegas containing only those components havinglow boiling points. This isoften undesirable in that the components having high boiling points whenleft in the gas add'to the heating value of the gas. Additionally, theapparatus required to carry out these prior methods are expensive toinstall and operate.

The present invention provides a method of removing only thosecomponents from a'gas stream which will condense at the conditions to beencountered within the transmission or distribution system therebyallowing a gas stream to be deliveredto the point of use having aheating value as high as possible;

heat with cool residue gas entering heat exchanger 12 through'conduit14. Heat exchangerlZ may be any conventional commercially available heatexchanger formed of materials of sufficient strength to withstandthe-pressure and temperature levels of the gas streams passingtherethrough. A shell and tube type of heat exchanger is preferred, withthe inlet gas stream passing through the tubes (the tube side) andthecool gas stream passing through the exchanger on the outside of ciallyavailable refrigerant such as Freon or Propane may be used in thepresent invention. The refrigerant is cooled through expansion acrossvalve 24 and upon entering chiller l8 exchanges heat with the wellstream passing through coil 20. A level controller 26 which may be anyconventional pneumatic or electric level control device maintains alevel of liquid refrigerant in chiller 18 above coil 20 by opening orclosing expansion valve 24. Expansion valve 24 may be any pneumatic orelectric control valve which will respond to the output signal of levelcontroller 26.

As a result of the expansion through valve 24 and the heat absorbed bythe refrigerant in chiller 18 from the well stream within coil 20, therefrigerant is vaporized and passes out of chiller 18 through conduit28. From conduit 28 the refrigerant vapors enter the suction side of aconventional refrigerant compressor 30 wherein they are compressed. Thecompressed vapors are conducted from compressor 30 through conduit 32 toa heat exchanger 34 wherein heat is removed from the refrigerant vaporscausing them to condense. The condensed refrigerant then passes throughconduit 36 back to expansion valve 24. Thus, a conventional closedrefrigeration circuit is provided whereby the refrigerant is vaporizedin chiller 18 removing heat from the well stream passing therethrough,compressed by compressor 30, condensed in heat exchanger 34 and expandedby expansion valve 24.

From chiller 18, the gas stream passes through conduit 38 into separator40. While the well stream passes through heat exchanger 12 and chiller18 wherein it is cooled, components contained in the gas stream arecondensed. By controlling the temperature of the well stream as itpasses through chiller 18, as will be further discussed, only thosecomponents required to obtain a desired residue gas hydrocarbon dewpoint are condensed. The residue gas and condensed components enterseparator 40 wherein the condensed components are separated from theresidue gas and removed. Separator 40 may be any conventional liquid-gasseparator which will bring about the separation of the condensed liquidfrom the residue gas. The condensed liquid accumulates in the bottomportion of separator 40 from where it enters conduit 42 and passesthrough liquid level control valve 44. A level controller 46, which maybe any conventional pneumatic or electric level controller senses thelevel of liquid in separator 40 and opens and closes valve 44accordingly. Valve 44 may be any conventional automatic control valvewhich will open and close in response to the signal received from levelcontroller 46. From control valve 44 the condensed components are passedinto conduit 48 from where they may be conducted to a storage tank orpoint of further processing.

A temperature controller 52, which may be any conventional pneumatic orelectric temperature controller is provided on separator 40 to sense thetemperature of the residue gas passing through separator 40. The outputsignal from temperature controller 52 operates an automatic unloadingdevice 54 which is connected to refrigerant compressor 30. Unloadingdevice 54 stops and starts, or otherwise loads or unloads compressor 30in response to a signal from temperature controller 52. Such loadingdevices are commonly provided with refrigerant compressors which arecommercially available. Thus, temperature controller 52 automaticallyloads compressor 30 thereby circulating more refrigerant through chiller18 if the residue gas in separator 40 becomes too warm, and willautomatically unloads compressor 30 thereby reducing the flow ofrefrigerant through chiller 18 if the residue gas in separator 40becomes too cold.

The residue gas separated from the condensed components in separator 40passes upwardly in separator 40 into conduit 50. From conduit 50 theresidue gas is conducted to conduit 14 from where it enters theshellside of heat exchanger 12 previously described. Heat is exchangedwith the inlet gas stream passing through the tube side of heatexchanger 12 thereby cooling the inlet gas stream and heating theresidue gas stream. The thus heated residue gas stream passes out ofheat exchanger 12 through conduit 56 from where it enters heat exchanger34. Heat exchanger 34 may be any conventional heat exchanger formed ofmaterials which will withstand the pressure and temperatures to beencountered. A shell and tube type of heat exchanger is preferred. Theresidue gas entering heat exchanger 34 passes through the tube side ofheat exchangers 34 and exchanges heat with compressed refrigerant vaporspassing through the shellside of heat exchanger 34. The heat transferredfrom the refrigerant vapors into the residue gas heats the residue gasand condenses the refrigerant vapors. The thus heated residue gas isconducted from heat exchanger 34 through conduit 58 to a gastransmission or distribution system.

OPERATION In operation, the apparatus for carrying out the method of thepresent invention is connected to a natural gas stream which is to betransmitted through a pipe line or transmission system to a point ofdistribution. The gas stream is cooled to a desired temperature, thedetermination of which will be discussed further below, by settingtemperature controller 52 to control the residue gas passing throughseparator 40 at that temperature. The inlet gas stream passes throughheat exchanger 12 wherein it is pre-cooled and then enters chiller 18where it is cooled to the desired temperature. When compressor 30 isloaded, it causes refrigerant to circulate within the closed refrigerantsystem. That is, condensed refrigerant enters expansion valve 24 fromconduit 36 whereupon the pressure level of the refrigerant is reducedcausing some vaporization of the refrigerant thereby cooling it. Heattransferred from the well stream passing through coil 20 disposed withinchiller 18 causes further vaporization of the refrigerant. The vaporizedrefrigerant accumulates in the top portion of cooler 18 from where itpasses into conduit 28. From conduit 28 the refrigerant vapors pass intocompressor 30 wherein the vapors are compressed to a relatively highpressure and pass through conduit 32 to heat exchanger 34. Upon passingthrough heat exchanger 34, the refrigerant is substantially allcondensed into a liquid. The liquid then passes into conduit 36 fromwhere it is recycled through expansion valve 24, etc.

Heat exchanger 34 accomplishes two important functions. First, it bringsabout the condensation of the refrigerant vapors in the manner describedabove, and second, along with heat exchanger 12, it brings about heatingof the residue gas to a temperature well above the hydrocarbon dew pointof the residue gas. The heated residue gas may then be conducted to atransmission or distribution system without fear of condensationoccurring.

DESIGN A typical natural gas stream produced from oil and gas wells isshown in Table I.

WELLS Component Composition MOL Nitrogen l Carbon Dioxide 0.50 Methane94.37 Ethane 3. l0 Propane 0.50 iso-Butane 0.10 n-Butane 0. l0iso-Pentane 0.04 n-Pentane 0.04 Hexanes 0.09 Heptanes 0.03 Octanes 0.03Total 100.00

Let it be assumed that a natural gas stream having the composition shownabove is to enter a transmission pipe line at a pressure of 1,000 poundsper square inch absolute. Additionally, let it be assumed that thepressure in the transmission pipe line will vary from a pressure of1,000 pounds per square inch absolute atthe forward end thereof to apressure of pounds per square inch absolute at the termination point orfinal gas distribution point. Also, let it be assumed that thetemperature of the gas stream while it travelsthrough the pipe line canget as low as 8F. due to atmospheric conditions. In order to preventcondensation in the transmission and distribution system, all componentscontained in the gas stream which will condense within the pressurerange of 15 to 1,000 pounds per square inch absolute and a temperatureof 8F. must be removed.

Referring to FIG. 2, curve 60 represents the hydrocarbon dew point ofthe gas stream having the composition given in Table I over the pressurerange of from 15 to 1,000 pounds per square inch absolute. As will beunderstood by those skilled in the art, the hydrocarbon dew point of thegas stream may be calculated at any particular pressure and temperatureusing known engineering techniques and curve 60 prepared accordingly. Ascan be seen from curve 60 of FIG. 2, the hydrocarbon dew point of thegas stream will be well above 8 throughout substantially theentirepressure range expected and condensation would occur in the pipeline. Thus, it is necessary to remove condensible components from thegas stream before it enters the pipe line in order to lower thehydrocarbon dew point of the residue gas below 8F.

In order to determine the temperature to which the inlet gas stream mustbe cooled to remove only those components required to reduce thehydrocarbon dew point of the residue gas to 8F. over the pressure rangeexpected, an operating temperature is assumed and the dew points of theresidue gas calculated over the pressure range expected. Referring againto FIG. 2, curve 62 represents the hydrocarbon dew points of the residuegas remaining if the inlet gas stream is cooled to 5F. As can be seenfrom the curve the maximum dew point of 8F. will occur at a pressure of600 pounds per square inch absolute. Thus, by cooling the gas streamhaving a composition as given in Table I to a temperature of 5F. at apressure of 1,000 pounds per square inch absolute, the residue gasremaining will not condense over the range of pressures and temperaturesexpected in the pipe line.

After the operating temperature is determined as described above, theapparatus for carrying out the method of the present invention may bedesigned in accordance with standard engineering practices to cool thegas stream to that temperature in the most economical manner. That is, acomparison of various sizes of v chillers 18, related refrigerationequipment and heat exchangers 12 may be made to arrive at the mosteconomical combination of these apparatus. As will be understood bythose skilled in the art, the heat exchange surface area of heatexchangers l2 and 34 and coil 12, and the sizes of chiller 18, separator40, and other equipment may be determined by applying conventionalengineering techniques'taking into consideration the volume of gas to beprocessed and the gas composition, pressure, temperature, etc.

Thus, the present invention provides a method of economically removingonly those components within a gas stream required to produce a residuegas having a hydrocarbon dew point which will not condense within aparticular transmission and distribution system.

The present invention therefore is well adapted to carry out the objectsand attain the ends and advantages mentioned as well as those inherenttherein. While presently preferred embodiments of the invention aregiven for the purpose of disclosure, numerous changes can be made whichwill readily suggest themselves to those skilled in the art and whichare encompassed within the spirit of the invention disclosed herein.

What is claimed is:

l. A method of controlling the hydrocarbon dew point of a gas stream tobe transported inc a pipeline system so that components of said gasstream are not condensed in said pipeline system comprising the stepsof:

cooling said gas stream with an expanded refrigerant stream to atemperature such that only those components which will condense in saidpipeline systems are liquefied and a residue gas stream having a highheating value remains; separating said residue gas stream from saidliquefied condensible components; compressing said expanded refrigerantstream in accordance with temperature changes in said residue gas streamso that the temperature of the residue gas stream is controlled at thedesired level; exchanging heat between said residue'gas stream and saidcompressed refrigerant stream to condense said refrigerant stream and toheat said residue gas stream to a temperature level above thehydrocarbon dew point of said residue gas stream; and conducting saidresidue gas stream to saidpipeline system. I

2. The method of claim I which is further character.- ized to includethe step of cooling said gas stream with said residue gas stream priorto cooling said gasstream with said expanded refrigerant stream.

3. The method of claim I wherein said refrigerant stream isconfinedwithin a closed refrigeration cycle.

4. A method of controlling the hydrocarbon dew point of a gas stream tobe transported in a pipeline system so that components of said gasstream are not condensed in said pipeline system comprising the stepsof:

passing said gas stream in heat exchange relationship with an expandedrefrigerant stream thereby vaporizing said refrigerant stream andcooling said gas stream to a temperature such that only those componentscontained therein which will 'condense in said pipeline are liquefiedand a residue gas stream having a high heating value remains;

separating said residue gas stream from said liquefied components;

compressing said expanded and vaporized refrigerant 10 stream inaccordance with temperature changes in said residue gas stream so thatthe temperature of the residue gas stream is controlled at the desiredlevel;

passing said residue gas stream in heat exchange relastream is confinedwithin a closed refrigeration cycle.

to said pipeline

2. The method of claim 1 which is further characterized to include thestep of cooling said gas stream with said residue gas stream prior tocooling said gas stream with said expanded refrigerant stream.
 3. Themethod of claim 1 wherein said refrigerant stream is confined within aclosed refrigeration cycle.
 4. A method of controlling the hydrocarbondew point of a gas stream to be transported in a pipeline system so thatcomponents of said gas stream are not condensed in said pipeline systemcomprising the steps of: passing said gas stream in heat exchangerelationship with an expanded refrigerant stream thereby vaporizing saidrefrigerant stream and cooling said gas stream to a temperature suchthat only those components contained therein which will condense in saidpipeline are liquefied and a residue gas stream having a high heatingvalue remains; separating said residue gas stream from said liquefiedcomponents; compressing said expanded and vaporized refrigerant streamin accordance with temperature changes in said residue gas stream sothat the temperature of the residue gas stream is controlled at thedesired level; passing said residue gas stream in heat exchangerelationship with said compressed refrigerant stream vapor so that saidresidue gas stream is heated and said compressed refrigerant streamvapor is condensed; and conducting said residue gas stream to saidpipeline system.
 5. The method of claim 4 which is further characterizedto include the step of passing said gas stream in heat exchangerelationship with said residue gas stream prior to passing said gasstream in heat exchange relationship with said expanded refrigerantstream.
 6. The method of claim 4 wherein said refrigerant stream isconfined within a closed refrigeration cycle.